MyPy complains if file replaced by shadow-file doesn't exist

[cpitclaudel]

This is a follow-up to #2119. I think this a bug report, but maybe it's a feature request, or even possibly a documentation request.

I'm surprised by the following behavior of --shadow-file SOURCE_FILE SHADOW_FILE:

$ cd /tmp/
$ mkdir python
$ cd python/
$ echo "x = 1 + 'A'" > shadow.py
$ mypy --shadow-file error.py shadow.py error.py #1
mypy: can't read file 'error.py': No such file or directory
$ mypy --shadow-file error.py shadow.py shadow.py #2
mypy: can't read file 'shadow.py': No such file or directory
$ mypy shadow.py #3
shadow.py:1: error: Unsupported operand types for + ("int" and "str")

I see two problems here:

• I expected # (1) to behave just as if there had been a file error.py with the contents of shadow.py, returning the same as # (3). This use case happens concretely when a user of Emacs creates a new file (but hasn't saved it to disk yet). In that case we create a temporary file with the contents of the unsaved file, and pass that as shadow.py, with error.py being the name of the yet-unsaved file.

• In # (2) I did not expected the --shadow-file option to make a difference, since I remapped error.py to shadow.py and then asked about shadow.py (which shouldn't be affected by the unrelated mapping). Additionally, the error seems wrong (the file shadow.py exists).

I'm using mypy 0.570 and Python 3.5.2 (I have not tried with mypy from git master)

Am I misunderstanding the way --shadow-file is used?

Thanks!

[gvanrossum]

Your invocation (1) is correct, but the error.py file should still exist (its existence is checked before the shadowing goes into effect, though its contents is irrelevant).

In (2) the shadow flag has no effect because you're not type-checking error.py.

You'll find that if you create an empty error.py, (1) will work correctly -- it will report the same error as (3), except the filename will be error.py.

[cpitclaudel]

Thanks!

In (2) the shadow flag has no effect because you're not type-checking error.py.

But it does have an effect: mypy produces a nonsensical error message. Is that intended?

You'll find that if you create an empty error.py, (1) will work correctly -- it will report the same error as (3), except the filename will be error.py.

I'm well aware of this :) But it's inconvenient. Is there a strong reason to require that the file exist?

[gvanrossum]

Oh, I wonder if you've got the arguments to --shadow-file reversed. See #4712

[cpitclaudel]

Sorry if I'm being slow. I'm struggling to reconcile the docs (and the behavior that I observe) with your comment. In your comment you say that the correct invocation is mypy --shadow-file flycheck_tt.py tt.py tt.py (flycheck_tt is the temporary one; tt is the real one). But the docs says this:

  --shadow-file SOURCE_FILE SHADOW_FILE
                            Typecheck SHADOW_FILE in place of SOURCE_FILE.

and indeed when I try thing in the order suggested by the docstring, everything works as expected:

$ echo '1 + "A"' > temp.py
$ echo '"no error"' > real.py
$ mypy --shadow-file real.py temp.py real.py 
real.py:1: error: Unsupported operand types for + ("int" and "str")

whereas in the order that you suggest, I don't get errors:

$ mypy --shadow-file temp.py real.py real.py 

My core issue is different, though; I'd like one of these (I'm not sure which one anymore :/) to work:

$ mypy --shadow-file temp.py nonexistent.py nonexistent.py
mypy: can't read file 'nonexistent.py': No such file or directory
$ mypy --shadow-file nonexistent.py temp.py nonexistent.py
mypy: can't read file 'nonexistent.py': No such file or directory 

[cpitclaudel]

… and I made the mistake of mixing two bugs in one in my original report — sorry about this. The following seems suspicious to me:

$ touch a.py
$ mypy --shadow-file a.py nonexistent.py a.py
mypy: can't read file 'a.py': No such file or directory
$ mypy --shadow-file nonexistent.py a.py a.py
mypy: can't read file 'a.py': No such file or directory

(I could understand mypy complaining about nonexistent.py, but it seems odd to complain about a.py, which does exists)

[gvanrossum]

This flag is just super confusing. Sorry.

…

On Fri, Mar 16, 2018, 15:59 Clément Pit-Claudel ***@***.***> wrote: … and I made the mistake of mixing two bugs in one in my original report — sorry about this. The following seems suspicious to me: $ touch a.py $ mypy --shadow-file a.py nonexistent.py a.py mypy: can't read file 'a.py': No such file or directory $ mypy --shadow-file nonexistent.py a.py a.py mypy: can't read file 'a.py': No such file or directory (I could understand mypy complaining about nonexistent.py, but it seems odd to complain about a.py, which does exists) — You are receiving this because you modified the open/close state. Reply to this email directly, view it on GitHub <#4746 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACwrMmmo4hRpi9fy0Q76aOC56zZqpwGnks5tfEPZgaJpZM4StQoG> .

[gvanrossum]

Also, if you want to help, you can put in some print statements (or better, use pdb) and try to find the bug(s) in the code.